Re: [PATCH v2 4/5] mm: FLEXIBLE_THP for improved performance

[Huang, Ying]

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Fri, Jul 07, 2023 at 01:29:02PM +0200, David Hildenbrand wrote:
>> On 07.07.23 11:52, Ryan Roberts wrote:
>> > On 07/07/2023 09:01, Huang, Ying wrote:
>> > > Although we can use smaller page order for FLEXIBLE_THP, it's hard to
>> > > avoid internal fragmentation completely.  So, I think that finally we
>> > > will need to provide a mechanism for the users to opt out, e.g.,
>> > > something like "always madvise never" via
>> > > /sys/kernel/mm/transparent_hugepage/enabled.  I'm not sure whether it's
>> > > a good idea to reuse the existing interface of THP.
>> > 
>> > I wouldn't want to tie this to the existing interface, simply because that
>> > implies that we would want to follow the "always" and "madvise" advice too; That
>> > means that on a thp=madvise system (which is certainly the case for android and
>> > other client systems) we would have to disable large anon folios for VMAs that
>> > haven't explicitly opted in. That breaks the intention that this should be an
>> > invisible performance boost. I think it's important to set the policy for use of
>> 
>> It will never ever be a completely invisible performance boost, just like
>> ordinary THP.
>> 
>> Using the exact same existing toggle is the right thing to do. If someone
>> specify "never" or "madvise", then do exactly that.
>> 
>> It might make sense to have more modes or additional toggles, but
>> "madvise=never" means no memory waste.
>
> I hate the existing mechanisms.  They are an abdication of our
> responsibility, and an attempt to blame the user (be it the sysadmin
> or the programmer) of our code for using it wrongly.  We should not
> replicate this mistake.
>
> Our code should be auto-tuning.  I posted a long, detailed outline here:
> https://lore.kernel.org/linux-mm/Y%2FU8bQd15aUO97vS@xxxxxxxxxxxxxxxxxxxx/

Yes.  Auto-tuning should be more preferable than any configuration
mechanisms.

Something like THP shrinker could be another way of auto-tuning.

https://lore.kernel.org/linux-mm/cover.1667454613.git.alexlzhu@xxxxxx/

That is, allocating the large folios on page fault, then try to detect
internal fragmentation.

>> I remember I raised it already in the past, but you *absolutely* have to
>> respect the MADV_NOHUGEPAGE flag. There is user space out there (for
>> example, userfaultfd) that doesn't want the kernel to populate any
>> additional page tables. So if you have to respect that already, then also
>> respect MADV_HUGEPAGE, simple.
>
> Possibly having uffd enabled on a VMA should disable using large folios,
> I can get behind that.  But the notion that userspace knows what it's
> doing ... hahaha.  Just ignore the madvise flags.  Userspace doesn't
> know what it's doing.

Best Regards,
Huang, Ying